This invention relates generally to prime movers and, more specifically, to controlling induction motors.
The AC induction motor is the workhorse of industry. AC induction motors are simple, rugged, and easily maintained. They are inexpensive to construct and because they are inexpensive they are the motor choice for 90% of all applications in industry.
By itself the AC induction motor has two shortcomings. First, the AC induction motor is not a true constant speed machine. The motor when turning under a load will always turn at a rotational frequency less then that of the rotational frequency of the magnetic field within the motor. Second, the motor is not by itself inherently capable of providing variable speed operation. Several factors indicate the speed of the turning motor including the frequency of the input power, the nature of the load and the current available. Both of these characteristics are the result of a phenomenon known as slip.
The AC induction motor includes two basic assemblies, the stator and the rotor. The stator assembly includes steel laminations shaped to form poles. Copper wire coils are wrapped around these poles to form primary windings to be connected to a voltage source thereby producing a rotating magnetic field. The rotor assembly is made of laminations formed around a steel shaft core. Radial slots around the laminations"" periphery have rotor bars, i.e. conductors shorted at the ends positioned parallel to the shaft. The arrangement of these rotor bars is reminiscent of an exercise wheel for pet rodents giving the configuration a nickname xe2x80x9csquirrel cagexe2x80x9d.
Torque within the induction motor is developed as the result of the interaction of currents induced in the rotor bars with the rotating magnetic field. Because the induction is necessary, in operation the rotor speed always lags behind the magnetic field speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. The different between the rotating speed of the magnetic field and the speed of the rotor is known as the slip speed and varies with load because of the resultant increase need for torque. Because of this variability of the produced speed based on input power of a constant frequency, precise control of the induction motor is difficult.
One solution is the adjustable speed control usually based on pulse width modulation. The constant alternating current (xe2x80x9cA.C.xe2x80x9d) line voltage of 60 or 50 cycles per second from the supply network is rectified, filtered, and then converted into a variable voltage at a variable frequency. Adjusting the frequency and voltage according to the needed output of the motor has proven to be a successful strategy for control. Using a load torque signal added to the speed controller allows slip compensation proportional to the load. When optimized, the slip can be reduced to about 0.5%.
The disadvantage of the adjustable speed control is that it is generally necessary to rectify alternating current to direct current before being able to generate variable frequency power. Doing so requires rectifying the A.C. line voltage to direct current (xe2x80x9cD.C.xe2x80x9d) and then xe2x80x9cchoppingxe2x80x9d the power according to the needed frequency. In each of the two stages inefficiencies are introduced making the use of the induction motor more costly over its life.
U.S. Pat. No. 6,466,468 issued to Douglas York on Oct. 15, 2002 presented a novel means of producing variable frequency power without rectifying that power to direct current. The teaching of the York patent, incorporated by this reference, is to chop power to a frequency significantly above the range of frequencies necessary to drive the motor for the application and by means of phase shifting the chopped power producing a power wave form represents the product of the input power at the input frequency multiplied by a reference sinusoid at an appropriate frequency to produce power of a desired frequency. This method is proved to be inherently more efficient then rectifying to direct current.
When uses motor controller however, the system taught in the York patent requires a one-to-one correspondence between the primary chopping phase and the secondary chopping phase. This requirement for one-to-one correspondence prevents the use of a single chopper with several secondary choppers for industrial applications needing more then a single induction motor. There is, therefore, an unmet need in the art for an efficient adjustable speed control for a motor allowing a one-to-many relationships between the primary chopper phase and the secondary chopper phase.
A programmable motor controller for receiving three-phase input power at an input power frequency and having a polarity is presented. The controller includes a primary chopper for reversing each phase of electrical power according to a reference frequency. Each primary chopper has a power input, a signal input, and an output. The power input is electrically connected in wye connection to each phase of power. A transformer for each phase of power has primary and secondary terminals and is connected electrically by its primary terminals to the output of the primary chopper. A secondary chopper for each phase of power is configured to reverse each phase or electrical power according to the reference frequency and phase shifted according to a second reference signal. The secondary chopper has an input connected electrically to the secondary terminals of the transformer in an output connected to a winding of a motor.
The invention modulates the, received input power by rapidly reversing its polarity at a frequency significantly higher then the frequency of the input power. Because the modulating frequency is significantly higher then the input frequency, the resulting waveform is a substantially square wave. Because the resulting waveform integrates to zero over time, use of the square wave avoids saturation of the isolation transformer. Additionally, because of the suitably high frequency, the transformers used for isolation can be much smaller than those used for conveying power at a grid frequency from 50 to 60 Hz.
The invention also allows more then one secondary chopper to be connected to the output of a single primary chopper, thereby allowing individual motor to be individually controlled within a single appliance without duplicating the hardware necessary for the primary chopper function.
The invention allows for the control of the frequency of the power fed to the motor and therefore the rotational frequency of the magnetic field within the stator. Appropriately controlling the rotational frequency of the magnetic field within the stator allows the optimization of the slip for efficiency.
The invention can also be used with synchronous or other AC machines and is not limited to induction motors though the advantages are most evident in conjunction with the induction machine.